In natural fibers to be used for clothes, such as wool and silk, variations of the structure and physical properties among individual constituent fibers are larger than those in synthetic fibers. Such a great variation is an advantage rather than a disadvantage taking the demand of clothes into consideration. Namely, woven and knitted fabrics of natural fibers have a higher bulkiness, a softer touch and a higher stiffness than those of synthetic fibers because of such variations. Accordingly, in the field of synthetic fibers, experiments have been intentionally done to manufacture mixed multifilament yarns by forming variations, such as differences in the denier and length among constituent filaments.
Conventionally in order to obtain such mixed multifilament yarn, there is ordinarily adopted a method in which at least two kinds of multifilament yarns differing in the physical properties such as shrinkage, elongation and elasticity are independently spun and the multifilament yarns are appropriately opened and mixed at the drawing step, the false-twisting step or other similar steps. This method is advantageous in that the length difference among constituent filaments can easily be adjusted by selecting each kind of multifilament yarn. However, when it is intended to obtain a fine-denier yarn having a fineness of less than 150 denier, especially less than 75 denier, which is eagerly desired at present as a material for a high-quality silky or woolly product, which was described above, it is necessary to spin at least two kinds of multifilament yarns having a much finer denier and then mix them. Generally speaking, the productivity is decreased as the denier of constituent filaments and the total denier is decreased. In addition, spinning and mixing steps are more troublesome than a single spinning step. Therefore, the yarn-manufacturing cost is increased and the method is not commercially practical.
As means for producing differences of the structures, physical properties, fineness and shapes among constituent filaments without causing a substantial increase of the yarn-manufacturing cost, there has been examined a method, wherein filaments are blended while they are being spun. For carrying out this method, there are two methods, i.e., a method using filaments of the same polymer, and a method comprising simultaneously spinning a plurality of polymers differing in their chemical structures, viscosities and the like.
In the former method wherein one polymer, is spun and blended, the extrusion amounts of the polymer are made different among extrusion holes of a spinneret, whereby there is obtained an assembly of filaments different in the deniers, sectional shapes and physical properties. In this method, however, since cooling of thick-denier filaments is delayed as compared with cooling of thin-denier filaments, there is obtained a multifilament yarn comprising a thick-denier component having low orientation and high elongation and a thin-component having high orientation and low elongation. If this multifilament yarn is subjected to a woolly treatment by utilizing a false twisting device, the thin-denier component is not easily elongated and is subjected to a high tension. As a result, fine-denier filaments gather in the central portion of the processed yarn. Contrary to this, since the thick-denier component is easily elongated, thick-denier filaments rise in the outer layer portion. A woven or knitted fabric formed from this yarn comes to have a hard touch and a weak stiffness. Namely, the knitted or woven fabric has a touch and feel quite different from those of a silky or woolly product for which a soft touch and a high stiffness are ordinarily required.
For the reasons set forth above, it is considered that the method of simultaneously spinning a plurality of polymers is the only one possible means left for attaining the above-mentioned mixed multifilament yarn. However, combinations of ordinary polymers which manifest substantial differences of physical properties under the same spinning conditions are in fact much more limited. Alternatively, use of special polymers involves a risk of deteriorating the feel and touch inherent in polyesters.